The invention relates to a coupling device with two parts which are coaxially rotatable relative to each other. The invention comprises a friction coupling whose friction plates are alternately connected to the parts which rotate relative to each other, and an actuating device for the friction coupling. The actuating device comprises an annular chamber formed in one of the parts, which contain a highly viscous fluid and the actuating device comprises a rotational member arranged in the annular chamber, drivingly connected to one of the parts, and which acts on the adhering fluid. The actuating device, when the rotatable parts rotate relative to each other, causes an increase in pressure for the purpose of loading the friction coupling.
When the rotatable parts of the coupling rotate relative to each other, the shearing action in the highly viscous fluid causes a pressure to be built up in the annular chamber. The pressure axially loads the friction plates of the friction coupling which thus have the tendency to close the coupling.
Devices of this type are used in motor vehicles and agricultural machinery to generate a differential-speed-dependent locking moment between two parts rotating relative to one another.
The two rotating parts may be parts of a differential drive or they may be connected to such parts of a differential drive. By using such an assembly in a differential drive, a limited slip differential which locks, i.e., has its differential action inhibited, as a function of the differential speed is produced.
A coupling device of said type is known from JP 7-17162 Y2 for example, wherein the rotational member is connected, so as to be rotationally fixed, to one of the relative rotating parts. One of the parts forms an annular chamber which is delimited by a displacable piston loading the friction coupling. With an increasing differential speed, the pressure build-up generated in the viscous fluid increases and causes the displacement of the piston.
A device of this type is also known from DE 195 05 800.3 wherein, for the purpose of limiting the pressure generated in the annular chamber, there exists a short circuit line between the annular chamber and reservoir. The short circuit line opens as a function of the pressure. However, the pressure-dependent control system can pass on the speed differential only indirectly. Thus, there may be interference due to the temperature of the viscous fluid. The channels required for the short circuit line, also render the device more expensive.
It is therefore the object of the present invention to provide a differential speed sensing coupling of this type which comprises simplified torque limiting means and which, in particular, do not require any additional housing bores.